With the exception of the brain, the liver is the organ most frequently examined by nuclear medicine procedures. Most of these studies involve the intravenous administration of labeled particles such as Tc-99m sulfur colloid which are effectively trapped by the Kupffer cells. The diagnostic information thus obtained is useful for studying liver morphology and Kupffer cell function. The optimum agent for these studies would have rapid, exclusive uptake into healthy Kupffer cells and would be biodegradeable and non-toxic.
Radionuclide studies are also used to measure hepatocyte function and bile duct patency (including gallbladder function). The optimum agent for these studies would have rapid, exclusive uptake by the hepatocytes, rapid intrahepatic transit, and prompt excretion into the biliary system. High specificity is required to obtain maximum diagnostic information while limiting patient radiation dose. The time to maximum liver concentration (t.sub.max) and the time for the liver concentration to decrease to 50% of the concentration at t.sub.max (t.sub.50) should both be short to reduce the time required for a study. This is important in patients that cannot be immobilized for long periods of time. Shorter study times increase the number of patients that can be handled in a given period of time maximizing the use of imaging equipment. A short t.sub.50 also means an increase in the ratio of radioactivity within the bile to that within the liver, thereby resolving the intrahepatic ducts from the liver and thus improving the quality of the image and of the resultant diagnosis.
Until recently, most of the nuclear medicine biliary function studies were performed with I-131 rose bengal or I-131 bromosulfophthalein. Although I-131 rose bengal is usually the standard for comparing liver function agents, its photon energy is not optimum for gamma camera imaging, and the patient exposure to non-diagnostic beta radiation limits the dose which can safely be administered. Image resolution and, therefore, diagnostic information is limited.
The development of Tc-99m penicillamine by Tubis e al (Radiopharmaceuticals, Subramanian et al, eds., The Society of Nuclear Medicine, Inc., New York, 1975, pgs 55-62) in 1974 was a significant step in overcoming the limitations of the iodine-131 label and in stimulating research interest in technetium-99m labeled hepatobiliary agents. Among the most useful technetium-99m labeled agents developed to date are the N-substituted iminodiacetic acids (hepatoiminodiacetic acids-HIDA's). Loberg et al., in U.S. Pat. No. 4,017,596 disclose, inter alia, the use of a chelate of technetium-99m and a substituted iminodiacetic acid for external organ imaging. Eckelman et al., in U.S. Pat. No. 3,725,295 disclose the labeling of diethylenetriaminepentaacetic acid (DPTA) with technetium-99m.
The various HIDA derivatives which have been disclosed in the literature can be represented by the formula ##STR2## The analogs discussed include the 2,6-dimethyl derivative (Loberg et al., U.S. Pat. No. 4,017,596); 2,6-diethyl, 2,6-diisopropyl, 2-butoxy, 4-butoxy, 4-butyl, 4-isopropyl, 4-ethoxy and 4-iodo derivatives (Wistow et al., Radiology, 128:793-794, 1978 and J. Nucl. Med., 18(5):455-461, 1977); 2,3-dimethyl, 2,4-dimethyl, 2,5-dimethyl, 3,4-dimethyl and 3,5-dimethyl derivatives (Van Wyk et al., Eur. J. Nucl. Med., 4:445-448, 1979); and 2,4,6-trisubstituted derivatives wherein at least two of the substituents are alkyl of 1 to 4 carbons, the third substituent is hydrogen or alkyl of 1 to 4 carbons, and together the substituents contain at least three carbons (Belgian patent No. 855,107); and 2,6-dimethyl, 2,6-diethyl, 2,6-diisopropyl, 4-methyl, 4-ethyl, 4-isopropyl, 4-n-butyl, 4-n-pentyl, 4-t-butyl, 4-phenyl, 4-methoxy, 3,5-dimethyl, 2,4,6-trimethyl, 2,4,5-trimethyl, 4-fluoro, 2,4-difluoro, 2,5-difluoro, and 2,3,4,5,6-pentafluoro derivatives (Molter et al., 3rd Int. Symp. Radiopharm. Chem., St. Louis, Mo., June 1980). Fields et al, Journal of Labelled Compounds and Radiopharmaceuticals, XV: 387-399 (1978) disclose N-(2-phenethylcarbamoylmethyl)iminodiacetic acid. p Van Wyk et al., Eur. J. Nucl. Med., 4:445-448, 1979, in their work with dimethyl HIDA derivatives have shown that steric effects of substituents are important for biliary uptake. Chiotellis et al., Int. J. Nucl. Med. Biol., 7:1-7, 1980, working with 4-butyl HIDA compounds, have shown that molecular size of substituents is important for biliary uptake.